DHA is one of n-3 highly unsaturated fatty acids that in recent years, have attracted attention as essential fatty acids having a wide variety of physiological activities. DHA has been known to have an antithrombogenic activity, an antiinflammatory activity, an antiallergic activity and an antitumor activity and to be important in developing and maintaining a nerve tissue of brain, retina or the like; it has therefore aroused interest. DHA is contained in a fish oil extracted from fish and shellfish in a relatively large amount. Such a DHA-containing fish oil has been marketed as a capsule-type healthy food formed by coating the same with gelatin or the like. Meanwhile, attempts have been made to add the same to general foods such as a beverage, a confectionery, a dairy dish, a seasoning and the like, and there are a large number of commercial products.
However, since DHA has a long chain and many double bonds and gives a peculiar smell derived from a fish oil, the purification, the concentration and the anti-oxidation thereof are technically difficult in many cases. As a result, a DHA-containing fish oil which is subjected to purification, emulsification, milling and anti-oxidation such as addition of an antioxidant is costly. For this reason, DHA is currently added to food only in an amount which is far from an average amount of DHA (from 0.5 g to 1.0 g per day) ingested by a Japanese from fish and shellfish. Further, in the conventional technique, after the DHA-containing fish oil emulsified or milled is added to a food, the stability of DHA is not maintained satisfactorily, so that such a food cannot be stored. Thus, the addition of DHA at a high concentration is prevented.
On the other hand, soybeans abundantly contain proteins having a high essential amino acid content and excellent in vivo absorbability, linoleic acid which is an n-6 highly unsaturated fatty acid, and functional substances such as lecithin and saponin having a cholesterol-decreasing activity owing to their emulsifiability. Further, soybeans, as well as garlic, onion and the like, have drawn attention as a food having an activity to prevent tumor. Tofu or soybean milk drink which is produced using such soybeans as a starting material naturally contains these useful substances abundantly, and is eaten and drunk every day as a traditional healthy food in Japan.
If DHA can be added to the above-mentioned tofu or soybean milk drink which is the typical healthy food in Japan, a person who does not eat fish frequently can naturally ingest DHA from a dairy meal or drink, and his health can be expected to thereby be maintained or promoted. It is considered that DHA is added to tofu or soybean milk drink by a method in which an emulsion having a high storage stability, which is obtained by emulsifying a DHA-containing fish oil using an appropriate emulsifying agent and adding a preservative such as sugar or salt at a high concentration to increase a storage stability, is added; a method in which the above-described emulsion is added as a dry powder; or a method in which a powder obtained by adsorbing a DHA-containing fish oil onto a porous carrier is added.
However, there has been no report indicating that DHA is added to tofu or soybean milk drink in such an amount that a Japanese can ingest DHA in a day from fish or shellfish, nor has the commercial product been marketed. The reason is that not only the above-described emulsion, its powder or porous support-adsorbed powder of the DHA-containing fish oil is expensive relative to the DHA content, thereby increasing the costs of the starting materials for tofu or soybean milk drink, but also the emulsifying agent, the preservative, the porous carrier and the like which are used influence the taste and the properties of tofu or soybean milk drink, thereby preventing the addition of DHA at a high concentration.